fix invalid job url in logs

[oleksandr-didyk]

Fix double slash in Helix URL in logs.

Example of invalid URL can be found here

[MattGal]

@oleksandr-didyk this change does not likely work the way you expected, as seen in this build:

  Sent Helix Job; see work items at [System.String]::Copy('https://helix.dot.net/').TrimEnd('/')/api/jobs/ec0681d6-dd31-40c9-9715-5d98c27cd44b/workitems?api-version=2019-06-17&access_token={Get this from helix.dot.net}
  Sending Job to (Debian.10.Amd64)Ubuntu.2004.Amd64@mcr.microsoft.com/dotnet-buildtools/prereqs:debian-10-helix-amd64...
  Sent Helix Job; see work items at [System.String]::Copy('https://helix.dot.net/').TrimEnd('/')/api/jobs/d260bcd3-74db-4d67-80ba-601322070b21/workitems?api-version=2019-06-17&access_token={Get this from helix.dot.net}
  Waiting for completion of job ec0681d6-dd31-40c9-9715-5d98c27cd44b on Windows.10.Amd64